In multimedia streaming a sequence of ‘moving images’ with sound is sent from a streaming server to a client device. In contrast to the technique in which an entire media file has to be arrived at the client before it can be played, the streaming technique enables the sending of media (video and/or audio) from the streaming server to the client in a continuous manner and the playing of the media as it arrives at the client.
A packet switched streaming service (PSS) is currently being standardized for mobile environment by 3GPP (3rd Generation Partnership Project). Compared to a fixed landline Internet environment, new problems specific to the mobile environment arise. These problems are mostly due to the different restrictions of mobile systems.
In mobile systems, information between network and mobile communications devices is transferred over a radio path, i.e. over an air-interface with the aid of radio frequency channels. The air-interface provides only limited radio resources (limited bandwidth) for communication. Accordingly, it is desired to use the limited bandwidth of the air-interface as efficiently as possible (so that radio resources are not wasted) in order to guarantee proper functioning of the system.
An example of a communications system capable of streaming media (streaming video and/or audio) is shown in FIG. 1. The system comprises a streaming server 111 which is coupled to an IP-network (Internet Protocol) 104. The IP-network 104 may be, for example, the Internet or a service provider operator's intranet (an intranet network belonging to the operator's domain). The IP-network 104 is coupled to a core network 103 of a mobile communications network via a Gi interface. The mobile communications network also has a radio access network (RAN) 102 coupled to the core network 103. The radio access network 102 provides mobile communications devices 101 with access to the mobile communications network over an air-interface. The mentioned access may be provided either by circuit switched means (circuit switched voice or data call) or packet switched means or both. In the following, GPRS (General Packet Radio Service) is used as an example of a packet switched means to communicate over the air-interface.
Streaming media is typically performed by sending pre-recorded (multi)media (video and/or audio) files from the streaming server 111 to a mobile communications device 101 (hereinafter referred to as a client device 101) in a compressed form. Depending on the codecs used to encode the media content, the streaming server may send the media to the client device 101 at a set of different bit rates. As an example, the server may have the content encoded at three bit rates. These bit rates may, for example, be produced by three different codecs or by one multi-rate codec. It should usually be the case that the higher the bit rate, the better the received picture and sound quality. However, a higher bit rate consumes more of the limited air-interface bandwidth.
The standardized GPRS Release '97 networks and GPRS Release '99 (EGPRS, Enhanced GPRS) networks use TDMA (Time Division Multiple Access) time slots for communication over the air-interface. The number of time slots together with the amount of bits used for error correction define the effective bandwidth for the payload of the connection. Accordingly, in order to enable the use the limited radio resources efficiently, there are different time slot and coding scheme (error correction) combinations defined for both GPRS Release '97 and Release '99 networks.
For example, GPRS Release '97 networks provide the following time slot and coding scheme possibilities (for up to 3 time slots):
bit rates [kbps]TS 1 + 1TS 2 + 1TS 3 + 1CS-19.0518.127.15CS-213.426.840.2
The table shows the effective air-interface downlink bandwidth (i.e. bit rate) available for payload (user data, useful data) depending on the time slot (TS) and coding scheme (CS) configuration. For example, in the configuration in which the used coding scheme is CS-1 and the used time slot configuration is TS 2+1 (2 time slots used in downlink direction (RAN->client device) and 1 time slot used in uplink direction (client device->RAN)), the available downlink bandwidth is 18.1 kbps. This is actually the effective available bandwidth for payload, for example, streaming (multi)media. Concerning GPRS, the generic term radio access network (RAN) is considered to comprise base (transceiver) stations (BTS) and base station controllers (BSC).
Correspondingly, GPRS Release '99 networks provide the following time slot and coding scheme possibilities (for up to 2 time slots):
bit rates [kbps]TS 1 + 2TS 2 + 2MCS-18.8017.6MCS-211.222.4MCS-314.829.6MCS-417.635.2MCS-522.444.8MCS-629.659.2MCS-744.889.6MCS-854.4108.8MCS-959.2118.4
Again, the table shows the effective air-interface downlink bandwidth available for payload depending on the time slot (TS) and coding scheme (MCS (Modulation and Coding Scheme)) configuration. For example, in the configuration in which the used coding scheme is MCS-6 and the used time slot configuration is TS 2+2 (2 time slots used in downlink direction and 2 time slot used in uplink direction) the available downlink bandwidth is 59.2 kbps.
It should be noted that the available air-interface downlink bandwidth for streaming media (i.e. the available air-interface downlink bit rate for streaming) may vary drastically during a streaming session. The radio access network 102 may, for example, have to increase air-interface error protection during a streaming session, if the quality of the received streaming media in the client device 101 drops due to changed air-interface conditions (bad radio link quality). Alternatively, or in addition, the radio access network (GPRS) may have to change the time slot configuration due to changed load conditions in the radio access network. Both of these situations may result in a change in the available air-interface bandwidth.
It should be noted that the air-interface bandwidth is a different concept than a server bandwidth (i.e. the bit rate on which the server sends the streaming media).
Let us consider an example, in which the original air-interface bandwidth for a streaming session is 59.2 kbps (GPRS Rel. '99: MCS-6 & TS 2+2) and it has been agreed, in streaming session setup, that the streaming server 111 sends at a bit rate 59 kbps. During the streaming session, the radio access network 102 then has to increase air-interface error protection due to changed air-interface conditions from MCS-6 to MCS-5. This results in a new time slot and coding scheme combination, namely: MCS-5 & TS 2+2. After the change, the available air-interface bandwidth is 44.8 kbps. If the streaming server continues sending at the bit rate of 59 kbps, this will result in larger delays and packet losses due to network buffer overflow since the air-interface can at most sustain the bit rate of 44.8 kbps which is considerably less than the server sending bit rate of 59 kbps. Ultimately, the streaming session may even be lost.
In order to overcome this problem, Ericsson has proposed a set of solutions in the following publications:                Ericsson, Improved Session Setup and Bandwidth Adaptation, 3GPP TSG-SA WG4 Meeting #17, Tdoc S4-010349, Jun. 4-8, 2001, Naantali, Finland.        Ericsson, Improved Session Setup and Bandwidth Adaptation, 3GPP TSG-SA WG4 Meeting #18, Tdoc S4-(01)0477, Sep. 3-7, 2001, Erlangen, Germany.        Ericsson, Proposal for Bandwidth Selection in PSS, 3GPP TSG-SA WG4 Meeting #22, Tdoc S4-(02)0407, Jul. 22-26, 2002, Tampere, Finland.        
According to the proposed solutions, the possible bit rates at which the streaming server 111 can send the streaming media are communicated beforehand to the client device 101. The server 111 or the client 101 may notice changed network conditions during an active session. If the server 111 notices changed network conditions, it may switch bit streams (this is understood such that the server 111 may switch from sending at a first bit rate to sending at another bit rate). If the client 101 notices changed network conditions, it may request a bit stream switch between the current bit stream and another bit stream (known to the client 101) by sending a particular message to the server 111. The server may then either accept or disregard the request for a bit stream switch.
In the example described in the foregoing, after the client device 101 changes the coding scheme from MCS-6 to MCS-5, it should send a request to the server 111 requesting the server to switch sending at a different bit rate. Since the available air-interface bandwidth after the change is 44.8 kbps, the client would most likely request the server to send at a bit rate next lowest to 44.8 kbps, for example 44 kbps as the case might be.
Whilst this solution may be a good one if there is an bit stream alternative close to 44.8 kbps, the situation is worse if the bit rate next lowest to 44.8 kbps is further away, for example, 30 kbps. Then, if the server 111 begins sending at 30 kbps, almost 15 kbps of air-interface bandwidth is wasted (not usable for streaming media). It is to be noted that using, in multimedia streaming, a bit rate almost 15 kbps lower than the one theoretically possible, will most probably negatively affect the quality of the picture and/or sound received at the client device 101.
Since there are and will be many different bit rates defined for the air-interface, as shown concerning the GPRS radio bearer in the tables in the foregoing, it is very unlikely that the streaming server 111 would have streaming media encoded at all the corresponding bit rates. Therefore, the scenario presented in the previous paragraph may well be realistic. Accordingly, there is a need for a new solution for coping with air-interface bandwidth variations without considerable waste of the available bandwidth.